Metal values can be recovered from different kinds of spent metal-containing compositions, obtained as residues from various processes, such as distillation, extraction, decanting and filtration. Spent catalyst compositions, like spent Fischer-Tropsch, hydrogenation, hydrotreating or reforming catalysts, are of a special interest.
Transition metals and their coordination complexes find various industrial applications as heterogeneous or homogeneous catalysts in petrochemical, pharmaceutical and fine chemical industries. Platinum Group Metals (PGM), Au, Ag, Co and Ni are examples of metals, which are used in a variety of applications like hydrogenation, oxidation, hydroformulation, carbonylation, etc.
The economical use of metal-based catalysts is always accompanied with the efficient recovery and recycling of the valuable metals. A lot of research effort has been made to find economical routes for the recovery processes. Usually, the recovery is based on pyrometallurgical or hydrometallurgical processes.
The spent catalysts most often contain impurities due to their use, which usually induces problems in the application of traditional refining methods. For example the presence of residual organics spoils hydrometallurgical processes by hindering the leaching of metals by the reduction of the active metallic surface area available by coverage.
Alternatively, when disposed of—meaning typically that the organic is removed by burning—the presence of coke on the surface may hinder traditional leaching processes and decrease the yield. In addition, burning off the organics may imply changes in the structure of the material, like the soluble γ-alumina turning into its insoluble α-form at high temperatures. Burning can also lead to valuable losses, like in the case of rhenium forming volatile Re2O7.
A specific spent catalyst composition is the one obtained from Fischer Tropsch syntheses used in the production of paraffin hydrocarbons or liquid fuel from syngas and hydrogen. The catalyst is an impregnated catalyst comprising an alumina carrier and catalytically active metals based on cobalt and/or iron and mixtures thereof. The catalyst may also comprise noble metals as promoters. Continuous processing causes the catalyst activity to drop, mainly due to sulphur impurities in the feed, which form either cobalt sulphide or precious metal sulphides on the surface of the catalyst. Coke formation may also occur and subsequently the catalyst is deactivated.
Recycling of spent Fischer Tropsch catalysts is a great challenge for the recycling industry since the spent catalysts usually contain a high content of paraffin (50-90%), and the catalyst particles vary in size, going down to a very fine powder. In addition, both base metals and precious metals must be recovered and refined.
Attempts to remove or significantly lower the amount of paraffin in the spent catalysts are described in patent publications US 2006/0111232, U.S. Pat. No. 6,974,842 and CN 1563280, which use solvents to extract the paraffin and filtration to recover the supported catalyst, which can be used to recover the valuable metals by traditional refining methods. Patent publication WO 2002/085508 additionally describes the fluidizing and oxidizing of the recovered catalysts powder, the reduction with a reducing gas to form a reduced catalyst powder, which is then mixed with hydrocarbons to form a regenerated slurry catalyst.
In principle, there are two possible ways for reclaiming the metals from a spent catalyst composition, which still contains organic contaminants:
1) a hydrometallurgical method by which the spent catalyst is digested and the metals selectively precipitated, and a 2) pyrometallurgical method by which the spent catalyst is incinerated, the metals melted out and the catalyst support slagged off. A major obstacle of the first method is a high sensitivity to the organic contaminant, and in fact the method usually requires an incineration step to remove the contaminant before leaching. The second method is less sensitive to pollutants but incineration at high temperatures may induce difficulties in consecutive hydrometallurgical steps wherein the metals are separated from each other.
A process for recovering the used Co-containing catalyst used in Fischer-Tropsch synthesis is presented in patent publication CN 1401427, which includes reducing at 800-1200° C. and 0.1-2 MPa in H2/N2 mixed gas flow, adding diluted nitric acid, dissolving, filtering, adding a solution of sodium hydroxide to obtain precipitate of cobalt hydroxide, filtering, adding diluted nitric acid, dissolving and evaporation crystallizing to obtain Co(NO3)2·6H2O. Its advantages are claimed to be high Cobalt recovery rate (more than 91%), high purity (more than 94%) and low cost.
Patent application WO 02/18663 A2 provides a process for the selective recovery of aluminium, cobalt and platinum from spent Fischer-Tropsch catalysts, by initially calcinating the mixture of the spent catalyst and sodium carbonate or the spent catalyst at different temperatures in the presence of air to oxidise the organic material. Following the calcination, metal oxides such as CoO and Co3O4, Co2AlO4 and Na2Al2O4 are formed. A solution of caustic soda is used, to selectively and efficiently dissolve aluminium oxide with substantially no dissolution of cobalt and platinum from the spent catalyst. The cobalt and platinum remain in the leached residue. The leached residue is dissolved in a solution of nitric acid to form cobalt nitrate, while the platinum remains in the acid residue, which finally is dissolved in aqua regia to form chloroplatinic acid.
It has now surprisingly been found that metals can be leached off selectively from spent catalyst compositions, without removal of the organic contaminant by incineration or other methods, by treating the spent catalyst with ionic liquids formed from ammonium compounds and a selection of hydrogen bond donors. On the other hand, imidazolium based ionic liquids can also be used as ionic liquids in the recovery process according to the invention. The ionic liquid, containing the metal value, can be separated from the organic contaminant, by separating said organic contaminant using an anti-solvent.
Ionic liquids have been extensively evaluated as environmentally friendly or “green” alternatives to conventional organic solvents for a broad range of applications. Ionic liquids have been known for over 50 years, and they were originally developed for applications as liquid electrolytes in electrochemistry. They are molten salts with melting points usually below 100° C. Ionic liquids consist of large organic cations and anions and in distinction to an ionic solution they are only composed of ions. There are a wide range of available anions and cations, which potentially give access to a broad variety of ionic liquids, which are liquid at temperatures below 100° C.
Ionic liquids show great potential as substitutes for common organic solvents, since for example miscibility, hydrophilicity and polarity can be adjusted to a reaction via an easy variation of the ion pairs. In addition, many ionic liquids have no detectable (or very low) vapour pressure and do not emit volatile organic compounds (VOC's), making them environmentally benign solvents.
The preparation of ionic liquids and their use has been described in several publications and patents. Recently, BASF introduced the Basil process—Biphasic Acid Scavenging Utilizing Ionic Liquids—as the world's first large industrial process to use ionic liquids (EP 1 472 201). With this process, acids can be removed rapidly and simply from reaction solvents. The reaction between acid and base creates a liquid salt rather than solid crystals, which in full-scale production could create problems. Reliance on ionic liquids eliminates time-consuming and expensive filtration. These liquids can easily be separated from the desired product by decanting.
Commercially available ionic liquids are mostly based on imidazoles, pyridines or ammonium compounds. Particularly interesting are the ionic liquids based on choline chloride, which is non-hazardous and used currently as chicken feed additive. Patent publications EP 1 324 979, EP 1 165 486 and EP 1 322 591 describe the preparation of ionic liquids based on choline chloride and their use. Ionic liquids of choline chloride find applications as solvents for peptide dissolution, separation and extraction of naturally occurring products, as solvents in various catalysed reactions, in recovery of platinum group metals (PGM's), zinc, copper and lead, as electrolytes for battery technology, photovoltaics and electrochromics, in electroplating metals like zinc, cobalt, copper, chromium, in electropolishing etc. In particular the dissolution of metal oxides like ruthenium oxide, copper (II) oxide, chromium (VI) oxide, vanadium pentoxide, lead (VI) oxide, manganese (IV) oxide and zinc oxide were found to be soluble in the ionic liquids prepared. The recovery of precious metals, in particular platinum and palladium, from materials in which they are present as oxides is also described. Examples of materials are for example spent automobile catalytic converters.
Ionic liquids are not “one size fits all” products. Each chemical reaction is different and therefore each process may require an ionic liquid tailored specifically to the task.